1. Field of the Invention
The present invention relates to a spin-transfer torque (STT) magnetic random access memory (MRAM), and, more particularly, to an STTMRAM element having magnetic tunnel junctions (MTJs) with a multi-layered free layer.
2. Description of the Prior Art
Magnetic random access memory (MRAM) is a type of non-volatile memory in which magnetization of magnetic layers in MTJs switches between parallel (corresponding to a low resistance state) and anti-parallel (corresponding to high resistance state) configurations. One type of MRAM is spin-transfer torque magnetic random access memory (STTMRAM) where switching occurs through the application of spin polarized current across the MTJ during programming.
STTMRAM has significant advantages over magnetic-field-switched (toggle) MRAM, which has been recently commercialized. The main hurdles associated with field-switched-MRAM are its more complex cell architecture with high write current (currently in the order of milliamps (mA)) and poor scalability attributed to the process used to manufacture these devices. That is, these devices cannot scale beyond 65 nanometer (nm) process node. The poor scalability of such devices is intrinsic to the field writing methods. The current generated fields to write the bits increase rapidly as the size of the MTJ elements shrink. STT writing technology allows directly passing a current through the MTJ, thereby overcoming the foregoing hurdles and resulting in much lower switching current [in the order of microamps (uA)], simpler cell architecture which results in a smaller cell size (for single-bit cells) and reduced manufacturing cost, and more importantly, improved scalability.
FIG. 1 shows a prior art STTMRAM element 10 having an anti-ferromagnetic (AFM) layer 6 on top of which is shown formed the a pinned layer (PL) (also known as a “fixed layer”) 5 on top of which is shown formed an exchange coupling layer 4 on top of which is shown formed a reference layer (RL) 3 on top of which is shown formed a barrier layer (BL, also known as a “tunnel layer” or a “MTJ junction layer”) 2 on top of which is shown formed a free layer (FL) (also known as a “storage layer (SL)”) 1. The layers 3-5 are typically referred to as “synthetic antiferromagnetic” (SAF) structure and generally used for providing reference to the free layer 1 during spin torque (ST) switching of the FL 1 and reading of the state of the FL 1 through the resistance down and across the element 10. The exchange coupling layer (ECL) 4 is typically made of ruthenium (Ru).
When electrons flow across the element 10, perpendicular to the film plane from the RL 3 to the FL 1, ST from electrons transmitted from the RL 3 to the FL 1 can orientate storage layer or free layer magnetization (as shown by the direction of the arrows in FIG. 1) to a direction parallel to that of RL 3. When electrons flow from the FL 1 to the RL 3, ST from electrons reflected from the RL 3 back into the FL 1 can orientate SL magnetization in a direction that is anti-parallel relative to that of RL 3. With controlling electron (current) flow direction, SL magnetization direction can be switched. Resistance across the element 10 changes between low and high resistance states when the magnetization of the FL 1 is parallel or anti-parallel relative to that of RL 3. However, the problem with the element 10 as well as other prior art STTMRAM elements is that the level of electric current required to switch the magnetization orientation of FL 1 between parallel and anti-parallel relative to that of RL 3 is still higher than a typical semiconductor CMOS structure can provide, therefore making prior art STTMRAMs' applicability to storage systems not practical.
What is needed is a STTMRAM element that can switch at lower current while still maintaining the same level of stability against thermal agitation.